There is a desire in the industry for higher circuit density in microelectronic devices made using lithographic techniques. One method of achieving higher area density is to improve the resolution of circuit patterns in resist films. It is known in the art that increasing the numerical aperture (NA) of the lens system of the lithographic imaging tool increases the resolution at a given wavelength. However, increasing the NA results in a decrease in the depth of focus (DOF) of the imaging radiation, thereby requiring a reduction in the thickness of the imaging resist film. Further, the industry-wide shift to shorter wavelength exposure systems also results in a decrease in the DOF. A decrease in the resist film thickness can lead to problems in subsequent processing steps (e.g., ion implantation and etching).
In order to overcome these problems, bilayer resists have been developed. Bilayer resists generally comprise a top thin film imaging layer coated on a thick organic underlayer. The resist is patterned by: (i) imagewise exposure and development of the top layer, and then (ii) anisotropically transferring the developed pattern in the top layer through the thick underlayer to the substrate. Suitably, the top layer contains precursors to refractory oxides such as silicon, boron, or germanium which enable the use of oxygen-reactive ion etching (RIE) in the image transfer step. However, the incorporation of silicon into the photoresist film often leads to the degradation of resolution and imaging performance.
Bilayer resists are known in the art. However, these resists were generally developed before the advent of deep U.V. lithography (e.g., 248 nm and 193 nm) and are of little utility for high-resolution imaging. For example, in the review article "Polymeric Silicon-containing Resist Materials", Advanced Material for Optics and Electronics, Vol. 4, pp. 95-127 (1994), there is disclosed on page 112 a positive bilayer resist having a top layer comprising the copolymer poly(co-trimethylsilylmethyl methacrylate and mono-oximido .alpha. diketone). The top layer is imaged by radiation chain scission and the image is transferred with oxygen R.I.E. However, the resist is not commercially viable due to slow photospeed and other resist performance problems. Therefore, there still is a need in the art for a bilayer photoresist suitable for commercial use.
It is therefore an object of the present invention to provide an improved bilayer photoresist.
Other objects and advantages will become apparent from the following disclosure.